Editing Openai/69186d20-2d14-8013-b9c6-c09000f173f6 (section)

===== In a deep, structural sense: =====
* Proto-Celtic developed a strong tendency to: - create geminate sonorants, and - allow fortis vs lenis contrasts to carry morphological weight.

From that common base:
* Goidelic branch (Old Irish → modern Irish/Scottish/Manx) - Geminate sonorants > fortis vs lenis distinctions (tense vs lax, not voiceless vs voiced). - Later mostly collapsed; double letters survive as historical markers.
* Brythonic branch (Welsh/Breton/Cornish) - Some sonorants, especially laterals and rhotics, drifted toward devoicing in certain contexts. - In Welsh this devoicing phonologised as ll = [ɬ], rh = [r̥], etc.

So the relationship is:

: 

Not:

: 

Different branches, parallel but independent exploitations of the same inherited “we like beefed-up sonorants” tendency.